Gas sensor control method

ABSTRACT

A catalytic conversion characteristic of a catalyst, which indicates a relationship between an air-to-fuel ratio and a catalytic conversion efficiency of the catalyst, includes a second air-to-fuel ratio point, which is a point of starting an outflow of NOx from the catalyst and is located on a rich side of a first air-to-fuel ratio point that forms an equilibrium point for a rich component and oxygen. A constant current circuit, which induces a flow of an electric current from an exhaust side electrode to an atmosphere side electrode through a solid electrolyte layer in a sensor element, is connected to the sensor element. A microcomputer controls a current value of the electric current, which is induced by the constant current circuit, based on a difference between the first air-to-fuel ratio point and the second air-to-fuel ratio point at the catalyst.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No. 14/454,792 filed on Aug. 8, 2014 and entitled GAS SENSOR CONTROL APPARATUS AND GAS SENSOR CONTROL METHOD, and is based on and incorporates herein by reference Japanese Patent Application No. 2013-167129 filed on Aug. 9, 2013.

TECHNICAL FIELD

The present disclosure relates to a gas sensor control apparatus and a gas sensor control method.

BACKGROUND

For instance, a gas sensor, which outputs an electromotive force, is provided at a vehicle engine (e.g., an automobile engine). In this type of gas sensor, exhaust gas, which is discharged from the engine, serves as a sensing subject of the gas sensor, and an oxygen concentration of the exhaust gas is sensed with the gas sensor. This type of gas sensor includes an electromotive force (EMF) cell, which outputs an electromotive force signal that varies depending on whether the exhaust gas is rich or lean. Specifically, when an air-to-fuel ratio is rich, the electromotive force cell outputs the electromotive force signal of about 0.9 V. In contrast, when the air-to-fuel ratio is lean, the electromotive force cell outputs the electromotive force signal of about 0 V.

In this type of gas sensor, when the air-to-fuel ratio of the exhaust gas changes between rich and lean, a change in the sensor output may be disadvantageously delayed relative to an actual change in the air-to-fuel ratio. In order to improve the output characteristic of such a gas sensor, various techniques have been proposed.

For instance, JP2012-063345A (corresponding to US2012/0043205A1) discloses a gas sensor control apparatus, in which a constant current circuit is connected to at least one of a pair of sensor electrodes (i.e., two sensor electrodes). In this gas sensor control apparatus, when it is determined that a demand for changing the output characteristic of the gas sensor is present, a flow direction of the constant electric current is determined based on the demand. Then, the constant current circuit is controlled to induce a flow of the constant electric current in the determined direction. Through the supply of the constant electric current, the output characteristic of the gas sensor is appropriately controlled.

The gas sensor (the O₂ sensor), which outputs the electromotive force, may be placed on a downstream side of a three-way catalyst in an exhaust conduit of the engine to sense rich/lean of the air-to-fuel ratio of the exhaust gas after purification of the exhaust gas with the three-way catalyst. In such a case, CO, HC (both being rich components) and NOx (lean component) of the exhaust gas can be purified with the three-way catalyst. However, a catalytic conversion efficiency of the three-way catalyst for converting, for example, NOx is rapidly deteriorated when the air-to-fuel ratio exceeds a predetermined air-to-fuel ratio around a theoretical air-to-fuel ratio (e.g., when the air-to-fuel ratio is located on a lean side of a catalytic conversion window of the three-way catalyst). According to the study conducted by the inventors of the present application, with respect to the catalytic conversion characteristic of the three-way catalyst, it is confirmed that an NOx outflow air-to-fuel ratio point, at which NOx begins to outflow from the three-way catalyst, is located on a rich side of an air-to-fuel ratio point, which forms an equilibrium point of the rich component and the oxygen at the three-way catalyst.

In view of the above points, even in the case where NOx outflows from the three-way catalyst on the downstream side of the three-way catalyst, the sensing of the lean based on the presence of the oxygen in the exhaust gas is not appropriately conducted at the gas sensor in some cases. Therefore, in the case where a control operation for limiting NOx emissions is carried out through use of the measurement signal of the gas sensor, undesirable NOx emissions may possibly unexpectedly occur.

In the prior art technique discussed above, the constant electric current is conducted between the two sensor electrodes to change the sensor output characteristic and thereby to improve the sensing response. Furthermore, at a high load operation of the engine, a response level is increased in view of an expected increase in the amount of NOx emissions in response to an increase in the amount of fresh air.

However, there is no prior art technique, which changes a sensor output characteristic in view of a catalytic conversion characteristic in the above described catalysts. It is considered that the technique, which is based on the new objective of changing the sensor output characteristic according to the catalytic conversion characteristic of the catalyst, has a significant meaning.

SUMMARY

The present disclosure is made in view of the above point. According to the present disclosure, there is provided a gas sensor control apparatus applied to an exhaust gas purifying device of an internal combustion engine, which includes a catalyst and a gas sensor. The catalyst is installed in an exhaust device of the internal combustion engine and purifies NOx, which is a lean component of an exhaust gas of the internal combustion engine, and a rich component of the exhaust gas. The gas sensor is installed at a location, which is in an intermediate portion of the catalyst or on a downstream side of the catalyst, to sense a gas component of the exhaust gas, which serves as a sensing subject, after purification of the exhaust gas with the catalyst. The gas sensor includes an electromotive force cell, which has a solid electrolyte body and a pair of electrodes, to generate an electromotive force in response to an air-to-fuel ratio of the exhaust gas. A catalytic conversion characteristic of the catalyst, which indicates a relationship between the air-to-fuel ratio and a catalytic conversion efficiency of the catalyst, includes a second air-to-fuel ratio point, which is a point of starting an outflow of the NOx from the catalyst and is located on a rich side of a first air-to-fuel ratio point that forms an equilibrium point for the rich component and oxygen. The pair of electrodes of the electromotive force cell includes a reference side electrode, which becomes a positive side at a time of outputting the electromotive force from the electromotive force cell, and an exhaust side electrode, which becomes a negative side at the time of outputting the electromotive force from the electromotive force cell. The gas sensor control apparatus includes a current conduction regulating device and a control device. The current conduction regulating device induces a flow of an electric current from the exhaust side electrode to the reference side electrode through the solid electrolyte body in the electromotive force cell. The control device controls a current value of the electric current, which is induced by the current conduction regulating device, based on a difference between the first air-to-fuel ratio point and the second air-to-fuel ratio point at the catalyst.

According to the present disclosure, there is also provided a gas sensor control method applied to the exhaust gas purifying device of the internal combustion engine discussed above. In the gas sensor control method, a current value of an electric current, which flows from the exhaust side electrode to the reference side electrode through the solid electrolyte body in the electromotive force cell, is set based on a difference between the first air-to-fuel ratio point and the second air-to-fuel ratio point at the catalyst, and a flow of the electric current from the exhaust side electrode to the reference side electrode through the solid electrolyte body in the electromotive force cell is induced based the set current value of the electric current.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.

FIG. 1 is a diagram schematically showing an entire structure of an engine control system according to an embodiment of the present disclosure;

FIG. 2 is a diagram schematically showing a cross section of a sensor element and a sensor control arrangement of the embodiment;

FIG. 3 is an electromotive force characteristic diagram indicating a relationship between an air-to-fuel ratio and an electromotive force of the sensor element;

FIG. 4 is a diagram showing catalytic conversion characteristics of a first catalyst and output characteristics of an O₂ sensor;

FIG. 5 is a diagram showing catalytic conversion characteristics of the first catalyst and output characteristics of an O₂ sensor;

FIG. 6 is a schematic diagram for describing reactions of gas components at the sensor element;

FIG. 7 is a flowchart showing a procedure of a constant electric current control operation of the embodiment;

FIG. 8 is a diagram used for setting a command current value of a constant current circuit in the embodiment;

FIG. 9 is a flowchart showing an abnormality determination process for the constant current circuit in the embodiment;

FIG. 10 is a diagram showing a test result with respect to the amount of NOx emissions and the amount of HC emissions on a downstream side of the first catalyst in the embodiment;

FIG. 11 is a diagram showing a structure of a sensor control arrangement in a first modification of the embodiment according to the present disclosure; and

FIG. 12 is a diagram showing a structure of a sensor control arrangement in a second modification of the embodiment according to the present disclosure.

DETAILED DESCRIPTION

An embodiment of the present disclosure will be described with reference to the accompanying drawings. In the present embodiment, a gas sensor, which is provided in an exhaust conduit of an engine (internal combustion engine) of a vehicle (e.g., an automobile), is used, and there will be described an engine control system, which executes various control operations of the engine based on an output of the gas sensor. In the engine control system, an electronic control unit (hereinafter referred to as an ECU) is used to execute, for example, a control operation of a fuel injection quantity and a control operation of ignition timing. FIG. 1 is a diagram that schematically shows an entire structure of the engine control system.

In FIG. 1, the engine 10 is, for example, a gasoline engine and has an electronically controlled throttle valve 11, fuel injection valves 12, and ignition devices 13. Catalysts (also referred to as catalytic converters) 15 a, 15 b, which serve as or forms an exhaust gas purifying device, are installed in an exhaust conduit 14 (serving as an exhaust device) of the engine 10. Each of the catalysts 15 a, 15 b is formed as, for example, a three-way catalyst. The catalyst 15 a is a first catalyst, which serves as an upstream side catalyst, and the catalyst 15 b is a second catalyst, which serves as a downstream side catalyst. As is well known in the art, the three-way catalyst purifies three noxious components of the exhaust gas, i.e., CO (carbon monoxide), HC (hydrocarbon) and NOx (nitrogen oxide, such as NO) and is formed by applying metal, such as platinum, palladium, rhodium, to a ceramic substrate that is configured into, for example, a honeycomb form or a lattice form. In this instance, at the three-way catalyst, CO and HC, which are the rich components, are purified through an oxidation reaction, and NOx, which is the lean component, is purified through a reduction reaction.

An air-to-fuel ratio (A/F) sensor 16 is placed on an upstream side of the first catalyst 15 a in a flow direction of the exhaust gas, and an oxygen (O₂) sensor 17 is placed between the first catalyst 15 a and the second catalyst 15 b, i.e., is placed on the downstream side of the first catalyst 15 a and on the upstream side of the second catalyst 15 b in the flow direction of the exhaust gas. The exhaust gas purifying device of the present disclosure may include the O₂ sensor 17 in addition to the first and second catalysts 15 a, 15 b. The A/F sensor 16 outputs an A/F signal, which is generally proportional to the air-to-fuel ratio of the exhaust gas. Furthermore, the O₂ sensor 17 outputs an electromotive force (EMF) signal, which varies depending on whether the air-to-fuel ratio of the exhaust gas is rich or lean.

Furthermore, various sensors, such as a throttle opening degree sensor 21, a crank angle sensor 22, an air quantity sensor 23 and a coolant temperature sensor 24, are installed in the engine control system. The throttle opening degree sensor 21 senses the opening degree of the throttle valve 11. The crank angle sensor 22 outputs a crank angle signal of a rectangular waveform at every predetermined crank angle (e.g., a period of 30 degree crank angle) of the engine 10. The air quantity sensor 23 senses the quantity of the intake air drawn into the engine 10. The coolant temperature sensor 24 senses the temperature of the engine coolant. Although not depicted in the drawings, besides the above sensors, there are also provided, for example, a combustion pressure sensor, which senses a combustion pressure in a cylinder of the engine, an accelerator opening degree sensor, which senses an opening degree of an accelerator (an accelerator manipulation amount or an amount of depression of an accelerator pedal), and an oil temperature sensor, which senses a temperature of an engine lubricating oil. These sensors respectively serve as an operational state sensing means.

The ECU 25 includes a microcomputer of a known type, which has a CPU, a ROM, and a RAM (memories). The ECU 25 executes various control programs, which are stored in the ROM, to perform various control operations of the engine 10 according to the engine operational state. Specifically, the ECU 25 receives signals from the above-described sensors, and the ECU 25 computes each corresponding fuel injection quantity and each corresponding ignition timing to execute, for example, the control operation for driving the fuel injection valves 12 and the control operation for driving the ignition devices 13 based on the signals.

Particularly, with respect to the fuel injection quantity control operation, the ECU 25 performs an air-to-fuel ratio feedback control operation based on a measurement signal of the A/F sensor 16, which is placed on the upstream side of the first catalyst 15 a, and a measurement signal of the O₂ sensor 17, which is placed on the downstream side of the first catalyst 15 a. Specifically, the ECU 25 executes a main feedback control operation in such a manner that an actual air-to-fuel ratio (an actual air-to-fuel ratio at the location on the upstream side of the first catalyst 15 a), which is sensed with the A/F sensor 16, coincides with a target air-to-fuel ratio, which is set based on the engine operational state. Also, the ECU 25 executes a sub-feedback control operation in such a manner that an actual air-to-fuel ratio (an actual air-to-fuel ratio at the location on the downstream side of the first catalyst 15 a), which is sensed with the O₂ sensor 17, coincides with a target air-to-fuel ratio. In the sub-feedback control operation, in view of, for example, a difference between the actual air-to-fuel ratio on the downstream side of the first catalyst 15 a and the target air-to-fuel ratio, the target air-to-fuel ratio used in the main feedback control operation is corrected, or a feedback correction amount used in the main feedback control operation is corrected. The ECU 25 executes a stoichiometric feedback control operation, which sets the target air-to-fuel ratio to a stoichiometric air-to-fuel ratio (theoretical air-to-fuel ratio), as the air-to-fuel ratio control operation.

Next, the structure of the O₂ sensor 17, which is placed on the downstream side of the first catalyst 15 a, will be described. The O₂ sensor 17 has a sensor element (also referred to as a sensing device) 31, which is configured into a cup shape. FIG. 2 shows a cross section of the sensor element 31. In reality, the sensor element 31 is configured such that the entire sensor element 31 is received in a housing or an element cover, and the sensor element 31 is placed in the exhaust conduit 14. The sensor element 31 serves as an electromotive force cell.

In the sensor element 31, a solid electrolyte layer (serving as a solid electrolyte body) 32 has a cup shaped cross section. An exhaust side electrode 33 is formed in an outer surface of the solid electrolyte layer 32, and an atmosphere side electrode 34 is formed in an inner surface of the solid electrolyte layer 32. Each of the electrodes 33, 34 is formed as a layer on the corresponding one of the outer surface and the inner surface of the solid electrolyte layer 32. The solid electrolyte layer 32 is an oxide sintered body, which conducts oxygen ions therethrough and is formed by completely dissolving CaO, MgO, Y₂O₃, and/or Yb₂O₃ as stabilizer into ZrO₂, HfO₂, ThO₂, and/or Bi₂O₃. Furthermore, each electrode 33, 34 is made of a noble metal, such as platinum, which has the high catalytic activity, and a surface of the electrode 33, 34 is covered with a porous coating that is chemically plated. The above-described two electrodes 33, 34 serve as a pair of electrodes (sensor electrodes). An inside space, which is surrounded by the solid electrolyte layer 32, is an atmosphere chamber (a reference gas chamber or simply referred to as a reference chamber) 35. A heater 36 is received in the atmosphere chamber 35. The heater 36 has a sufficient heat capacity to activate the sensor element 31, and the sensor element 31 is entirely heated by a heat energy, which is generated from the heater 36. An activation temperature of the O₂ sensor 17 is, for example, 500 to 650 degrees Celsius. The atmosphere gas (atmosphere air) is introduced into the atmosphere chamber 35, so that the inside of the atmosphere chamber 35 is maintained at a predetermined oxygen concentration.

In the sensor element 31, the exhaust gas is present at the outside (the electrode 33 side) of the solid electrolyte layer 32, and the atmosphere gas (atmosphere air) is present at the inside (the electrode 34 side) of the solid electrolyte layer 32. An electromotive force is generated between the electrode 33 and the electrode 34 in response to a difference in an oxygen concentration (a difference in an oxygen partial pressure) between the outside (the electrode 33 side) of the solid electrolyte layer 32 and the inside (the electrode 34 side) of the solid electrolyte layer 32. Specifically, the generated electromotive force varies depending on whether the air-to-fuel ratio is rich or lean. In such a case, the oxygen concentration at the exhaust side electrode 33 is lower than the oxygen concentration at the atmosphere side electrode 34, which serves as a reference side electrode, and the electromotive force is generated at the sensor element 31 while the atmosphere side electrode 34 and the exhaust side electrode 33 serve as a positive side and a negative side, respectively. In this instance, the exhaust side electrode 33 is grounded through an electric path 50 b, as shown in FIG. 2. Thus, the O₂ sensor 17 outputs the electromotive force signal, which corresponds to the oxygen concentration (the air-to-fuel ratio) of the exhaust gas.

FIG. 3 is an electromotive force characteristic diagram showing a relationship between the air-to-fuel ratio of the exhaust gas and the electromotive force of the sensor element 31. In FIG. 3, the axis of abscissas indicates a percentage of excess air λ. When the percentage of excess air λ is 1 (i.e., λ=1), the air-to-fuel ratio is the stoichiometric air-to-fuel ratio (theoretical air-to-fuel ratio). The sensor element 31 has the characteristics of that the electromotive force generated from the sensor element 31 varies depending on whether the air-to-fuel ratio is rich or lean, and the electromotive force generated from the sensor element 31 rapidly changes around the stoichiometric air-to-fuel ratio. Specifically, the electromotive force (also referred to as a sensor electromotive force) of the sensor element 31 at the rich time is about 0.9 V, and the electromotive force of the sensor element 31 at the lean time is about 0 V.

In FIG. 2, a sensor control arrangement (also referred to as a sensor control apparatus) 40 is connected to the sensor element 31. When the electromotive force is generated at the sensor element 31 in response to the air-to-fuel ratio (the oxygen concentration) of the exhaust gas, the sensor measurement signal (the electromotive force signal), which corresponds to the electromotive force generated at the sensor element 31, is outputted from the sensor element 31 to a microcomputer 41 of the sensor control arrangement 40 through an electric path 50 a. The microcomputer 41 has a CPU, a ROM, and a RAM (memories), and executes various operations upon execution of each corresponding program stored in one or more of the memories. The microcomputer 41 computes the air-to-fuel ratio based on the electromotive force signal of the sensor element 31. The sensor control arrangement 40 is formed in the ECU 25 of FIG. 1. At the ECU 25, the microcomputer 41 is formed as a computing device (computing means) that has an engine control function and a sensor control function. In this case, the microcomputer 41 computes the engine rotational speed and the intake air quantity based on the measurement results of the various sensors discussed above. However, instead of having the single microcomputer, the ECU 25 may be constructed to have an engine control microcomputer, which executes the engine control function, and a sensor control microcomputer, which executes the sensor control function, if desired.

Furthermore, the microcomputer 41 determines an activated state of the sensor element 31 and controls the driving operation of the heater 36 through a drive device 42, which is connected to the heater 36 through an electric path 50 c, based on a result of determination of the activated state of the sensor element 31. The technique of the activation determination of the sensor element 31 and the technique of the heater control are already known. Therefore, the activation determination of the sensor element 31 and the heater control will be briefly described. The microcomputer 41 periodically changes the voltage or the electric current applied to the sensor element 31 in a manner that is similar to an alternating current and senses a thus generated change in the electric current or a thus generated change in the electric voltage. A resistance of the sensor element 31 (an impedance of the sensor element 31) is computed based on the change in the electric current or the change in the voltage, and the energization control operation of the heater 36 is executed based on the resistance of the sensor element 31. At that time, there is a correlation between the activated state of the sensor element 31 (the temperature of the sensor element 31) and the resistance of the sensor element 31. When the resistance of the sensor element 31 is controlled to a predetermined target value, the sensor element 31 is held in the desired activated state (the state, under which the activation temperature of the sensor element 31 is held in a range of 500 to 650 degrees Celsius). For example, a sensor element temperature feedback control operation may be executed as the heater control operation.

When the engine 10 is operated, the actual air-to-fuel ratio of the exhaust gas is changed. For example, the air-to-fuel ratio may be repeatedly changed between rich and lean. At the time of changing the actual air-to-fuel ratio between rich and lean, when a deviation exists between the output of the O₂ sensor 17 and the presence of NOx, which is the lean component, the emission performance may possibly be influenced. For example, the amount of NOx in the exhaust gas may possibly be increased beyond the intended amount at the time of operating the engine 10 under the high load (the time of accelerating the vehicle).

In the present embodiment, a sensing mode of the O₂ sensor 17 is changed based on the relationship between the output characteristic of the O₂ sensor 17, which outputs the electromotive force, and the catalytic conversion characteristic of the first catalyst 15 a, which is placed on the upstream side of the O₂ sensor 17. Details of the change of the sensing mode of the O₂ sensor 17 will be described later. FIG. 4 is a diagram that shows the catalytic conversion characteristics of the first catalyst 15 a, which is the three-way catalyst, and the output characteristics of the O₂ sensor 17. Specifically, FIG. 4 shows:(I) a relationship between a catalytic conversion efficiency of each of the three noxious components (i.e., CO, HC, NOx) of the exhaust gas at the first catalyst 15 a and the air-to-fuel ratio; (II) a relationship between the gas concentration of each of the three noxious components and the oxygen on the downstream side of the first catalyst 15 a and the air-to-fuel ratio; (Ill) a relationship between the gas concentration of each of the three noxious components and the oxygen around the surface of the exhaust side electrode 33 of the O₂ sensor 17 and the air-to-fuel ratio; and (IV) a relationship between the electromotive force output of the O₂ sensor 17 and the air-to-fuel ratio.

The first catalyst (the three-way catalyst) 15 a has a catalytic conversion window, in which the catalytic conversion efficiency of each of the three noxious components becomes high around the point of the stoichiometric air-to-fuel ratio (percentage of excess air λ=1), as is known in the art. Furthermore, with respect to the concentrations of the three noxious components and the concentration of the oxygen on the downstream side of the first catalyst 15 a, it is understood that a reaction equilibrium point A1, at which the concentrations of the rich components (CO, HC) and the concentration of the oxygen become generally equal to one another, is present around the point of the stoichiometric air-to-fuel ratio, and an NOx outflow point A2, at which NOx (NO) begins to outflow from the first catalyst 15 a on the downstream side of the first catalyst 15 a, is also present. In this case, the NOx outflow point A2 (the point of starting the outflow of NOx from the catalyst 15 a) is located on the rich side of the reaction equilibrium point A1, and the NOx outflow point A2 and the reaction equilibrium point A1 are spaced from each other by a difference ΔA. That is, the first catalyst 15 a has the catalytic conversion characteristic of that the NOx outflow point (serving as a second air-to-fuel ratio point) A2, at which NOx begins to outflow from the first catalyst 15 a, is located on the rich side of the reaction equilibrium point (serving as a first air-to-fuel ratio point) A1, which forms the equilibrium point for the rich components and the oxygen. The reaction equilibrium point A1 is an inflection point of the equilibrium characteristic of the rich components and the oxygen, and the NOx outflow point A2 is an inflection point of the outflow concentration characteristic of NOx.

The reason for the generation of the deviation (difference) between the point A1 and the point A2 may be as follows. In the case where the exhaust gas, which contains CO, HC, NOx, and O₂, is guided to the first catalyst 15 a during the operation of the engine 10, NOx may possibly outflow from the first catalyst 15 a in addition to CO and HC. For example, even in the range of the catalytic conversion window of the three-way catalyst, it will be noted that some amount of CO, HC, and NOx outflows from the first catalyst 15 a when the amount of CO, HC, and NOx is precisely measured. In such a case, although O₂ outflows from the first catalyst 15 a in equilibrium with CO and HC (starting of the outflow of O₂ at the concentration of CO and HC≈0), NOx outflows from the first catalyst 15 a on the downstream side thereof regardless of the reaction of CO and HC. Therefore, the difference exists between the point A1 and the point A2.

Furthermore, the concentrations of the above three components and the oxygen around the exhaust side electrode 33 of the O₂ sensor 17 are the same as the concentrations of the above three components and the oxygen on the downstream side of the first catalyst 15 a. In this case, the amount of the rich components (CO, HC) is larger than the amount of oxygen on the rich side of the point A1, and the amount of oxygen is larger than the amount of the rich components on the lean side of the point A1. Therefore, in terms of the electromotive force of the O₂ sensor 17, one of a rich signal (0.9 V) and a lean signal (0 V) is outputted on one side or the other side of the reaction equilibrium point Al of the first catalyst 15 a. In this case, it can be said that the reaction equilibrium point for the rich components and the oxygen at the O₂ sensor 17 coincides with the reaction equilibrium point A1 at the first catalyst 15 a. Furthermore, NOx is present on the rich side of the point A1.

At the exhaust side electrode 33 of the O₂ sensor 17, the oxidation reaction and the reduction reaction of CO, HC and NOx of the exhaust gas take place according to the following chemical reaction formulae (1) to (3). CO+0.5O₂→CO₂   (1) CH₄+2O₂→CO₂+2H₂O   (2) CO+NO→CO₂+0.5N₂   (3)

Furthermore, there is established a relationship of k1, k2>>K3 where k1, k2 and k3 denote an equilibrium constant of the chemical reaction formula (1), an equilibrium constant of the chemical reaction formula (2), and an equilibrium constant of the chemical reaction formula (3), respectively.

In this case, at the O₂ sensor 17, the equilibrium point (the point at which the electromotive force output=0.45 V) is determined through the gas reactions of, for example, CO, HC NOx, and O₂. However, due to the differences in the equilibrium constant, the reactions of CO and HC with O₂ become main reactions at the exhaust side electrode 33.

Furthermore, the above difference ΔA is present in the catalytic conversion characteristic of the first catalyst 15 a, and the above difference ΔA has the influence on the output characteristic of the O₂ sensor 17. Therefore, in some cases, even when NOx outflows from the first catalyst 15 a, the output of the O₂ sensor 17 may not correspond to the outflow of NOx from the first catalyst 15 a. Thus, the outflow of NOx from the first catalyst 15 a cannot be correctly monitored, and thereby the amount of NOx emissions may possibly be increased.

In view of the above disadvantage, according to the present embodiment, the electric current, which has a predetermined current value, is conducted between the electrodes 33, 34 of the sensor element 31 of the O₂ sensor 17, so that at the location around the exhaust side electrode 33 of the O₂ sensor 17, the concentrations of the rich components are reduced, and the concentration of the oxygen is increased. Specifically, as shown in FIG. 5, the equilibrium point of the gas reaction around the exhaust side electrode 33 of the O₂ sensor 17 is changed from the point A1 to a point A3. In FIG. 5, in comparison to FIG. 4, all of the concentration characteristics of CO, HC and O₂ around the exhaust side electrode 33 of the O₂ sensor 17 are shifted to the rich side. In this way, in the case where the output characteristic of the O₂ sensor 17 is changed, and NOx outflows from the first catalyst 15 a, the output of the O₂ sensor 17 can correspond to the outflow of NOx.

The principle of inducing the change in the sensor output characteristic through conduction of the electric current between the electrodes 33, 34 is as follows. As shown in FIG. 6, CO, HC, NOx and O₂ are present around the exhaust side electrode 33 of the O₂ sensor 17. Under such a circumstance, the electric current is conducted through the sensor element 31 such that the oxygen ions are moved from the atmosphere side electrode 34 to the exhaust side electrode 33 through the solid electrolyte layer 32. Specifically, the oxygen pumping is executed at the sensor element 31. In this case, at the exhaust side electrode 33, the oxygen, which is moved to the exhaust side electrode 33 side through the solid electrolyte layer 32, reacts with CO and HC to form CO₂ and H₂O, respectively. In this way, CO and HC are removed around the exhaust side electrode 33, and the equilibrium point of the gas reaction around the exhaust side electrode 33 of the O₂ sensor 17 is shifted to the rich side.

Next, the structure of the sensor control arrangement 40, which executes the control operation of the O₂ sensor 17, will be described. The structure of the sensor control arrangement 40 is one shown in FIG. 2. Specifically, the sensor control arrangement 40 includes the microcomputer 41, which serves as a control device (or control means). The microcomputer 41 obtains the electromotive force signal, which is outputted from the sensor element 31, through, for example, an analog-to-digital (A/D) converter, and the microcomputer 41 computes the air-to-fuel ratio (particularly, the air-to-fuel ratio on the downstream side of the first catalyst 15 a) of the exhaust gas based on the obtained electromotive force signal. Furthermore, a constant current circuit (serving as a current conduction regulating device or current conduction regulating means) 43 is connected in the electric path, which electrically connects between the atmosphere side electrode 34 of the sensor element 31 and the microcomputer 41. More specifically, the constant current circuit 43 is connected through an electric path 80 to a portion 50 a 1 of the electric path 50 a that electrically connects between the atmosphere side electrode 34 of the sensor element 31 and the microcomputer 41. The portion 50 a 1 of the electric path 50 a is an intermediate location between the atmosphere side electrode 34 of the sensor element 31 and the microcomputer 41 in the electric path 50 a. The constant current circuit 43 is configured such that when the sensor element 31 generates the electromotive force, the constant current circuit 43 induces a flow of a predetermined constant electric current through the sensor element 31.

The constant current circuit 43 is configured to enable the flow of the constant electric current Ics from the exhaust side electrode 33 to the atmosphere side electrode 34 through the solid electrolyte layer 32 in the sensor element 31. Furthermore, the constant current circuit 43 has a pulse width modulation (PWM) drive device, and thereby the constant current circuit 43 can adjust a current value of the electric current through a PWM control operation (a duty control operation). Here, it should be noted that the term “current value” refers to a value of current (more specifically, electric current) and is also called as “current quantity” (i.e., a quantity of electric current) or “current amount” (i.e., the amount of electric current), and the current value may be expressed in units of, for example, amperes (A) or the like. The microcomputer 41 sets the current value of the constant electric current of the constant current circuit 43 (the current value of the electric current conducted through the constant current circuit 43) based on a demand for conducting the electric current and controls the constant current circuit 43 to induce the flow of the constant electric current Ics, which has the set current value.

In the present embodiment, the control operation of the constant electric current is executed based on a difference between the reaction equilibrium point A1 of the oxygen outflow at the first catalyst 15 a and the NOx outflow point A2 of the NOx outflow at the first catalyst 15 a. Particularly, the constant electric current is controlled such that the equilibrium point of the gas reaction around the exhaust side electrode 33 of the O₂ sensor 17 is placed at the NOx outflow point A2 or a point adjacent to the NOx outflow point A2. In this way, the output characteristic of the O₂ sensor 17 is changed based on the catalytic conversion characteristic of the first catalyst 15 a. Thereby, when NOx outflows from the first catalyst 15 a, the lean signal is outputted at the O₂ sensor 17 from the beginning of the outflow of NOx from the first catalyst 15 a.

Here, in view of ensuring the robustness of the O₂ sensor 17 for the purpose of limiting the NOx emissions, it is desirable that the equilibrium point of the gas reaction around the exhaust side electrode 33 of the O₂ sensor 17 is placed on the rich side of the NOx outflow point A2 (see FIG. 5). Specifically, the equilibrium point of the gas reaction around the exhaust side electrode of the O₂ sensor 17 may be shifted from the NOx outflow point A2 on the rich side of the NOx outflow point A2 by the amount of, for example, about 0.1 to 0.5% (more desirably 0.1 to 0.3%) in terms of the percentage of excess air λ to have a slightly rich state.

When the operational state of the engine 10 is changed, the amount of the rich components in the exhaust gas is changed. Specifically, when the rotational speed of the engine is increased, or when the load of the engine is increased, the amount of the rich components in the exhaust gas is increased. In other words, when the rotational speed or the load of the engine is increased, the flow rate of the rich gas is increased, and the gas concentration of the rich gas is increased. In such a case, when the current value of the electric current to be supplied to the sensor element 31 is kept constant regardless of the engine operational state, the equilibrium point of the gas reaction around the exhaust side electrode 33 of the O₂ sensor 17 may possibly be unintentionally deviated from a desirable position, which is set with reference to the NOx outflow point A2. That is, the amount of supplied oxygen, which is supplied around the exhaust side electrode 33 of the sensor element 31 by the supply of the electric current through the sensor element 31, may possibly fall short relative to the amount of the rich components around the exhaust side electrode 33 of the sensor element 31. When this shortage of the supplied oxygen occurs, the rich components remain around the exhaust side electrode 33. Thereby, the output characteristic of the O₂ sensor 17 cannot be changed in the desirable manner.

Therefore, in the present embodiment, the current value of the constant electric current conducted through the sensor element 31 (the current value of the constant electric current of the constant current circuit 43) is variably controlled based on the operational state of the engine 10. In this case, even when the amount of the required oxygen, which is required to have the equilibrium reaction of the rich gas on the surface of the exhaust side electrode 33 at the O₂ sensor 17, is changed in response to the engine operational state, the output characteristic of the O₂ sensor 17 can be changed in the desirable manner in response to the change in the amount of the required oxygen. An engine rotational speed, an engine load and/or a load rate of the engine may be used as a parameter(s) of the engine operational state.

Furthermore, in the case where the output characteristic of the O₂ sensor 17 is changed by the current value of the electric current of the constant current circuit 43, as discussed above, when an abnormality occurs in the constant current circuit 43, the exhaust emission performance is influenced. Therefore, in the present embodiment, an abnormality determining function (abnormality determining arrangement), which executes abnormality determination of a determination subject, i.e., the constant current circuit 43, is added to the microcomputer 41.

As shown in FIG. 2, as a structure that is used to sense the abnormality, a shunt resistor 45 for sensing the electric current is connected to the exhaust side electrode 33 (more specifically, the shunt resistor 45 being installed in the electric path 50 b, which connects between the exhaust side electrode 33 and the ground), and the electric current, which flows through the shunt resistor 45, is sensed with a current sensing device 46. The current sensing device 46 may include a differential amplifier circuit, which has, for example, an operational amplifier. In this case, the actual current value of the electric current (also referred to as actual electric current), which is conducted by the constant current circuit 43, is sensed with the shunt resistor 45 and the current sensing device 46, and the microcomputer 41 executes the abnormality determination for determining whether the abnormality is present in the constant current circuit 43 based on the actual current value of the electric current.

Next, a constant current control operation and an abnormality determination process, which are executed by the microcomputer 41, will be described with reference to flowcharts of FIGS. 7 and 9. FIG. 7 is the flowchart showing the constant current control operation. This operation is repeated by the microcomputer 41 at predetermined time intervals.

In FIG. 7, at step S11, it is determined whether an execution condition for executing the constant current control operation is satisfied. For instance, the execution condition may include the followings: (I) the O₂ sensor 17 and the constant current circuit 43 are both normal; and (II) the sub-feedback control operation is currently executed. When the answer to the inquiry at step S11 is YES, the operation proceeds to step S12.

At step S12, the engine operational state, such as the engine rotational speed and/or the engine load (e.g., the amount of the intake air), is obtained. Thereafter, at next step S13, a command current value of the constant electric current Ics is set based on the engine operational state, which is obtained at step S12. At this time, the command current value of the constant electric current is set based on, for example, a relationship shown in FIG. 8. In FIG. 8, when the engine rotational speed or the engine load is increased, the command current value of the constant electric current is increased.

Thereafter, at step S14, the control (control for conduction of electric current) of the constant current circuit 43 is executed such that the constant current circuit 43 induces the flow of the constant electric current, which has the current value set at step S13, through the sensor element 31.

FIG. 9 shows the flowchart indicating the abnormality determination process of the constant current circuit 43. This process is repeated at predetermined time intervals by the microcomputer 41.

In FIG. 9, at step S21, it is determined whether an execution condition for executing the abnormality determination process is satisfied. For example, this execution condition may include a condition of that the sensor element 31 is in the activated state, i.e., the temperature of the sensor element 31 is equal to or higher than the predetermined activation temperature. When the answer to the inquiry at step S21 is YES, the operation proceeds to step S22.

At step S22, the command current value of the electric current for the constant current circuit 43 at this time is obtained, and an actual current value of the electric current, which is sensed with the shunt resistor 45 and the current sensing device 46 at this time, is obtained. At this time, in a case where the command current value has been changed immediately before the time of executing step S22, the actual current value of the electric current should be sensed with the shunt resistor 45 and the current sensing device 46 and is obtained by the microcomputer 41 from the current sensing device 46 after stabilization of the actual current value of the electric current. Specifically, the actual current value is sensed with the shunt resistor 45 and the current sensing device 46 and is obtained by the microcomputer 41 from the current sensing device 46 after elapsing of a predetermined time period from the time of changing the command current value.

Then, at step S23, the command current value and the actual current value are compared with each other, and it is determined whether a difference (absolute value of the difference) between the command current value and the actual current value is smaller than a predetermined determination value K. When it is determined that the difference (absolute value of the difference) between the command current value and the actual current value is smaller than the determination value K at step S23, the operation proceeds to step S24 where it is determined that the constant current circuit 43 is normal. In contrast, when it is determined that the difference (absolute value of the difference) between the command current value and the actual current value is equal to or larger than the determination value K at step S23, the operation proceeds to step S25 where it is determined that the constant current circuit 43 is abnormal. Desirably, the determination value K is set in view of a circuit tolerance (e.g., a tolerance of a sensor IC). When it is determined that the constant current circuit 43 is abnormal at step S25, the operation proceeds to step S26. At step S26, there is executed a fail-safe operation, such as stopping of the conduction (supply) of the constant electric current through the constant current circuit 43, stopping of the sub-feedback control operation of the air-to-fuel ratio, turning on of an abnormality warning lamp provided in, for example, an instrument panel, and/or storing of diagnosis data in a storage device (e.g., a memory).

The present embodiment discussed above provides the following advantages.

The constant current circuit 43 is provided to induce the flow of the constant electric current from the exhaust side electrode 33 to the atmosphere side electrode 34 through the solid electrolyte layer 32 in the sensor element 31. The current value of the electric current, which is conducted by the constant current circuit 43, is controlled based on the difference between the reaction equilibrium point A1 (the first air-to-fuel ratio point) and the NOx outflow point A2 (the second air-to-fuel ratio point) at the first catalyst 15 a. With this construction, the output characteristic of the O₂ sensor 17 can be adjusted relative to the corresponding air-to-fuel ratio, at which NOx begins to outflow from the first catalyst 15 a. That is, in the case where NOx outflows from the first catalyst 15 a, the O₂ sensor 17 can generate the corresponding electromotive force, which corresponds to the outflow of NOx from the first catalyst 15 a. Therefore, the output characteristic of the O₂ sensor 17 can be appropriately changed, and thereby the NOx emissions can be limited.

The current value of the electric current, which is conducted by the constant current circuit 43, is controlled to shift the equilibrium point of the gas reaction around the exhaust side electrode 33 of the O₂ sensor 17 to the NOx outflow point A2 (the second air-to-fuel ratio point) or the point adjacent to the NOx outflow point A2. Thereby, it is possible to implement the more appropriate structure for limiting the emissions of NOx through use of the output of the O₂ sensor 17.

Particularly, when the current value of the electric current, which is conducted by the constant current circuit 43, is controlled in such a manner that the equilibrium point of the gas reaction around the exhaust side electrode 33 of the O₂ sensor 17 becomes slightly rich relative to the NOx outflow point A2 (the second air-to-fuel ratio point), the required robustness can be achieved to limit the NOx emissions.

FIG. 10 indicates a test result with respect to the amount of emissions of NOx and the amount of emissions of HC on the downstream side of the first catalyst 15 a. In FIG. 10, there is indicated the result of the air-to-fuel ratio control operation executed in the state where the output characteristic of the O₂ sensor 17 is changed. In FIG. 10, “A1” indicates the reaction equilibrium point A1 at the first catalyst 15 a, and “A2” indicates the NOx outflow point A2, and “λshift amount” indicates a rich change amount in terms of λ (percentage of excess air). In this case, it should be understood that the amount of NOx emissions is reduced in a range where the λ shift amount is 0 to 0.5%. That is, in FIG. 10, the amount of NOx emissions is advantageously reduced in the case where the equilibrium point of the gas reaction around the exhaust side electrode 33 relative to the NOx outflow point A2 is shifted within the predetermined range of the percentage of excess air λ, which is 0 to 0.5%, on the rich side of the NOx outflow point A2. Furthermore, an appropriate value of the λ shift amount is 0.1 to 0.5% (the appropriate value of the λ shift amount being in the predetermined range of the percentage of excess air λ, which is 0.1 to 0.5%). Desirably, the current value of the electric current to be conducted through the sensor element 31 is a value in a rage of 0.1 mA to 1.0 mA.

The current value (the command current value) of the electric current conducted by the constant current circuit 43 is controlled based on the engine operational state. In this way, even when the amount of the rich components in the exhaust gas is changed due to the change in the engine operational state, the output characteristic of the O₂ sensor 17 can be appropriately changed, and thereby the equilibrium point of the gas reaction around the exhaust side electrode of the O₂ sensor 17 can be maintained at the desired position relative to the NOx outflow point A2.

The present disclosure is not necessarily limited to the above embodiment, and the above embodiment may be modified in various ways within the principle of the present disclosure. For example, the above embodiment may be modified as follows.

The current conduction regulating device (current conduction regulating means) of the above embodiment may be constructed to have a structure shown in FIG. 11. In FIG. 11, there is provided a constant current circuit 50 as the current conduction regulating device (current conduction regulating means). The constant current circuit 50 conducts the constant electric current in response to the electromotive force upon application of the electromotive force of the sensor element 31 to the constant current circuit 50. The constant current circuit 50 includes a voltage generating arrangement 51, an operational amplifier 52, an n-channel MOSFET 53 and a resistor 54. The voltage generating arrangement 51 generates a predetermined constant voltage. The MOSFET 53 is driven by an output of the operational amplifier 52. The resistor 54 is connected to a source of the MOSFET 53. The MOSFET 53 and the resistor 54 are connected in series in an electric path 80, which connects between the portion 50 a 1 of the electric path 50 a and a ground. The electric path 50 a and the electric path 80 cooperate together to form an electric path 70 that connects between the atmosphere side electrode 34 of the sensor element 31 and the ground (earth). The voltage generating arrangement 51 includes a constant voltage source 51 a (e.g., a constant voltage source that outputs 5 V) and two resistors 51 b, 51 c. The constant voltage source 51 a and the resistors 51 b, 51 c are connected in series, and an intermediate point between the resistors 51 b, 51 c forms a voltage output point X1. A non-inverting terminal (also referred to as a + input terminal) of the operational amplifier 52 is connected to the voltage output point X1, and an output terminal of the operational amplifier 52 is connected to a gate of the MOSFET 53. Furthermore, an inverting terminal (also referred to as a − input terminal) of the operational amplifier 52 is connected to an intermediate point X2 between the MOSFET 53 and the resistor 54 in the electric path 80. The gate, the drain and the source of the MOSFET 53 are connected to the output terminal of the operational amplifier 52, the atmosphere side electrode 34 of the sensor element 31 and the resistor 54, respectively.

The constant current circuit 50, which is constructed in the above-described manner, is operated such that the voltage of the non-inverting terminal of the operational amplifier 52 and the voltage of the inverting terminal of the operational amplifier 52 become equal to each other. Therefore, the voltage of the intermediate point X2 and the voltage of the voltage output point X1 are equal to each other. The constant electric current Ics, which has a current value determined based on the voltage of the intermediate point X2 and the resistance value of the resistor 54, flows through a series circuit that is formed in the electric path 70 and includes the sensor element 31, the MOSFET 53 and the resistor 54 that are connected one after another in series. At this time, the MOSFET 53 is operated in response to an output voltage of the operational amplifier 52, which is generated due to a difference between the input voltage of the non-inverting terminal and the input voltage of the inverting terminal of the operational amplifier 52, so that the MOSFET 53 functions as a current control element that conducts the constant electric current Ics.

Desirably, the voltage of the voltage output point X1, the voltage of the intermediate point X2 and the resistance value of the resistor 54 are determined based on the current value of the electric current that needs to be conducted through the sensor element 31 at the time of generating the electromotive force in the sensor element 31. Specifically, for instance, in a case where the electric current, which has the current value of 0.1 mA, needs to be conducted through the sensor element 31 at the time of generating the electromotive force (0 to 0.9 V) in the sensor element 31, the voltage of the voltage output point X1 and the voltage of the intermediate point X2 are set to be 10 mV, and the resistance value of the resistor 54 is set to be 100 Ω. Furthermore, in a case where the electric current, which has the current value of 0.2 mA, needs to be conducted through the sensor element 31, the voltage of the voltage output point X1 and the voltage of the intermediate point X2 are set to be 20 mV, and the resistance value of the resistor 54 is set to be 100 Ω. In addition, in a case where the electric current, which has the current value in a range of 0.1 mA to 1.0 mA, needs to be conducted through the sensor element 31, when the resistance value of the resistor 54 is set to be 100 Ω, the voltage of the voltage output point X1 and the voltage of the intermediate point X2 may be set to be in a range of 10 mV to 100 mV. However, in such a case, the reference voltage, which is the voltage of the intermediate point X2 between the MOSFET 53 and the resistor 54 in the constant current circuit 50, is smaller than the electromotive force (0.45 V) of the sensor element 31 generated at the stoichiometric air-to-fuel ratio. Desirably, the range of the resistance value of the resistor 54 is about 50 Ω to 500 Ω.

In the sensor control arrangement 40, which has the constant current circuit 50 of the above structure, when the electromotive force is generated in the sensor element 31, the predetermined constant electric current Ics is conducted through the MOSFET 53 and the resistor 54 while using the electromotive force of the sensor element 31 as an electric power source (in other words, the sensor element 31 being used as a battery). Thereby, the output characteristic of the O₂ sensor 17 can be changed.

Furthermore, the microcomputer 41 changes, for example, the voltage value (the voltage value at the point X1) of the constant voltage of the voltage generating arrangement 51 based on the engine operational state. In this way, the current value of the electric current conducted through the sensor element 31 (the current value of the electric current conducted through the constant current circuit 50) is variably controlled. In this case, similar to the above embodiment, even when the amount of required oxygen, which is required to have the equilibrium reaction of the rich gas on the surface of the exhaust side electrode 33 at the O₂ sensor 17, is changed in response to the engine operational state, the output characteristic of the O₂ sensor 17 can be changed in a desirable manner in response to the change in the required oxygen.

Alternatively, the current conduction regulating device (current conduction regulating means) of the above embodiment may be constructed to have a structure shown in FIG. 12. In FIG. 12, a resistor circuit 60 is provided as the current conduction regulating device (current conduction regulating means). The resistor circuit 60 conducts the electric current in response to the electromotive force of the sensor element 31 upon application of the electromotive force of the sensor element 31 to the resistor circuit 60. The resistor circuit 60 includes a plurality of electric paths 80 a-80 c, each of which connects between the atmosphere side electrode 34 of the O₂ sensor 17 and the ground (earth), i.e., each of which connects between the electric path 50 a and the ground (earth). The electric paths 80 a-80 c and the electric path 50 a cooperate together to form the electric path 70 that connects between the atmosphere side electrode 34 of the sensor element 31 and the ground (earth). A series circuit of a switch 61 a-61 c and a resistor 62 a-62 c is formed in each of these electric paths 80 a-80 c in the resistor circuit 60. In FIG. 12, these series circuits include three series circuits. In each of these three series circuits, a corresponding one of the switches 61 a-61 c is provided as the switch, and a corresponding one of the resistors 62 a-62 c is provided as the resistor. Each of the switches 61 a-61 c is placed between the atmosphere side electrode 34 of the sensor element 31 and a corresponding one of the resistors 62 a-62 c. Each of the switches 61 a-61 c is a switch that is formed by a semiconductor switching element, such as a metal oxide semiconductor field effect transistor (MOSFET). Each of the switches 61 a-61 c is turned on or off in response to a command outputted from the microcomputer 41. Resistance values R1, R2, R3 of the resistors 62 a-62 c are different from each other. For instance, the resistance values R1, R2, R3 may be set to be, for example, R1>E2>R3.

In such a case, the switches 61 a-61 c are individually on/off controlled. Depending on which one of the switches 61 a-61 c is turned on, the resistor to be connected to the sensor element 31 is changed among the resistors 62 a-62 c to change the resistance value in the resistor circuit 60. The microcomputer 41 controls the turning on/off of each switch 61 a-61 c based on the engine operational state. In this way, the current value of the electric current conducted through the sensor element 31 (the current value of the electric current conducted through the resistor circuit 60) is variably controlled. In this case, similar to the above embodiment, even when the amount of required oxygen, which is required to have the equilibrium reaction of the rich gas on the surface of the exhaust side electrode 33 at the O₂ sensor 17, is changed in response to the engine operational state, the output characteristic of the O₂ sensor 17 can be changed in a desirable manner in response to the change in the required oxygen.

Here, it should be noted that the resistance values of the resistors 62 a-62 c may be set to be equal to each other (i.e., R1=R2=R3), if desired. In such a case, the resistance value of the resistor circuit 60, which is connected to the sensor element 31, may be changed depending how many of the switches 61 a-61 c is turned on (i.e., the number of the on-state switch(es) among the switches 61 a-61 c).

Furthermore, a voltage circuit, which applies a positive voltage to the exhaust side electrode 33, may be provided in the electric path 50 b between the exhaust side electrode 33 of the sensor element 31 and the ground. This voltage circuit may be an offset voltage circuit that raises, i.e., increases an electric potential of the exhaust side electrode 33 by a predetermined amount (predetermined electric potential) relative to an electric potential of the output side of the constant current circuit 43, from which the electric current flows out of the constant current circuit 43. In such a case, the electromotive force outputted from the sensor element 31 has a voltage that is obtained by adding the predetermined offset voltage to the voltage of the original sensor electromotive force (0 to 0.9 V) of the sensor element 31. This construction is particularly advantageous in a case where the sensor element 31 is used as the battery as in the case of FIG. 11 or FIG. 12. For instance, in the sensor control arrangement 40 of FIG. 11, in the case where the electromotive force of the sensor element 31 changes in the range of 0 to 0.9 V, when the electromotive force becomes small, the current value of the electric current, which is induced by the constant current circuit 50 and is conducted through the sensor element 31, may possibly become smaller than the proper current value. In view of this point, by installing the voltage circuit between the exhaust side electrode 33 and the ground, even in the relatively small electromotive force range, it is possible to limit the excessive reduction of the current value of the electric current, which is induced by the constant current circuit 50 and is conducted through the sensor element 31. Thereby, the output characteristic of the O₂ sensor 17 can be appropriately changed.

In the above embodiment (see FIG. 2), the constant current circuit 43 is connected to the atmosphere side electrode 34 of the sensor element 31. This construction may be modified. That is, the constant current circuit 43 may be connected to the exhaust side electrode 33. Alternatively, the constant current circuit 43 may be provided to both of the two electrodes 33, 34.

In the above embodiment, the O₂ sensor 17 is placed on the downstream side of the first catalyst 15 a. Alternatively, the O₂ sensor 17 may be installed to an intermediate portion of the first catalyst 15 a. In such a case, the O₂ sensor 17 may be installed to the substrate of the first catalyst 15 a. In any of the above cases, it is only required that the O₂ sensor 17 is constructed to use the exhaust gas after the purification thereof at the first catalyst 15 a as the sensing subject to sense the gas component(s).

Besides the O₂ sensor 17 having the above-described structure, the gas sensor may be a gas sensor that has a two-cell structure, which includes an electromotive force cell and a pump cell. In such a case, the output characteristic can be appropriately changed at the electromotive force cell of the two-cell type gas sensor. 

What is claimed is:
 1. A gas sensor control method applied to an exhaust gas purifying device of an internal combustion engine, which includes: a catalyst that is installed in an exhaust device of the internal combustion engine and purifies nitrogen oxide (NO_(x)), which is a lean component of an exhaust gas of the internal combustion engine, and hydrocarbon (HC) and carbon monoxide (CO), which are rich components of the exhaust gas; and a gas sensor that is installed at a location, which is in an intermediate portion of the catalyst or on a downstream side of the catalyst, to sense a gas component of the exhaust gas, which serves as a sensing subject, after purification of the exhaust gas with the catalyst, wherein: the gas sensor includes an electromotive force cell, which has a solid electrolyte body and a pair of electrodes, to generate an electromotive force in response to an air-to-fuel ratio of the exhaust gas; a catalytic conversion characteristic of the catalyst, which indicates a relationship between the air-to-fuel ratio and a catalytic conversion efficiency of the catalyst, includes a second air-to-fuel ratio point, which is a point of starting an outflow of the NO_(x) from the catalyst and is located on a rich side of a first air-to-fuel ratio point that forms an equilibrium point for the rich components and oxygen, while the rich side of the first air-to-fuel ratio point is a side of the first air-to-fuel ratio point at which concentrations of HC and CO serving as the rich components are increased in the exhaust gas in comparison to a lean side of the first air-to-fuel ratio point, and the lean side of the first air-to-fuel ratio point is a side of the first air-to-fuel ratio point at which a concentration of the NO_(x) serving as the lean component is increased in the exhaust gas in comparison to the rich side of the first air-to-fuel ratio point; the pair of electrodes of the electromotive force cell includes a reference side electrode, which becomes a positive side at a time of outputting the electromotive force from the electromotive force cell, and an exhaust side electrode, which becomes a negative side at the time of outputting the electromotive force from the electromotive force cell, the gas sensor control method comprising: setting a current value of an electric current, which flows from the exhaust side electrode to the reference side electrode through the solid electrolyte body in the electromotive force cell, based on a difference between the first air-to-fuel ratio point and the second air-to-fuel ratio point at the catalyst to place an equilibrium point of a gas reaction around the exhaust side electrode of the electromotive force cell at the second air-to-fuel ratio point or an adjacent point that is adjacent to the second air-to-fuel ratio point, wherein the equilibrium point of the gas reaction around the exhaust side electrode of the electromotive force cell is an equilibrium point for the rich components and the oxygen around the exhaust side electrode of the electromotive force cell; and inducing a flow of the electric current from the exhaust side electrode to the reference side electrode through the solid electrolyte body in the electromotive force cell, based on the set current value of the electric current. 